Ozone's Problem
with Polar Stratospheric Clouds

Using a GIF animation,
this page describes in detail the processes by which ozone is destroyed
over Antarctica by polar stratospheric clouds during the austral spring.

Introduction

The Antarctic ozone hole is produced every spring over the earth's south
pole because of the special conditions that are present in the stratosphere
over Antarctica and the presence of completely anthropogenic chemicals
spilled into the environment called chlorofluorocarbons (CFCs).

Unique Meteorology and Polar Stratospheric Clouds

One of the major factors producing the special chemistry that occurs in
the stratosphere there is the cold temperatures present during the Antarctic
winter. The air in the stratosphere is completely in the dark during the
austral winter. Antarctica is also totally surrounded by water (as contrasted
with the north pole which has landed geographic features on many sides
around it) and therefore meteorologically isolated from air at higher latitudes
during the winter. This physical condition forms an isolated air mass swirling
over Antarctica called the south polar vortex. In the air of the
polar vortex, the temperatures drop to very low levels in the stratosphere,
below 80 degrees below zero C. At these temperatures, chemicals present
in the stratosphere freeze out and form polar stratospheric
clouds (PSCs). It is the chemical reactions that occur on
the PSCs that result in the large decrease in ozone during each austral
spring over Antarctica that we called the Antarctic ozone hole.

Chlorofluorocarbons

The relative
chemical inertness of CFCs is one of the major reasons for they were
used in air conditioners, refrigerators, foam blowing agents, and as solvents
for cleaning circuit boards. Medical inhalers for asthmatics still use
chlorofluorocarbons as an inert carrier for the method of aerosol dispersion
necessary for the quick dispensing of medicine to people who need the drugs
quickly and effectively sprayed into their lungs. This is one of the few
allowed exemptions for CFCs.

CFCs TROPOSPHERIC inertness is based
on the fact that there are few gas phase tropospheric components that will
destructively react with them in the layer of the atmosphere closest to
the earth. Furthermore the wavelengths of light-that pass through the gases
in the upper atmosphere and get down to the troposphere-are not short enough
(read energetic enough) to photolytically decompose chlorofluorocarbons.

This chemical inertness means that chlorofluorocarbons have long atmospheric
lifetimes and become well-mixed throughout the troposphere. James Lovelock,
the inventor of the most sensitive analytical detector for CFCs-the
electron capture detector sailed into the southern Pacific ocean (1971-72)
on the RRS Shackleton-far away from the majority of CFC polluting sources
in the northern hemisphere (mainly the United States and Western Europe).
In his travels through the marine troposphere he detected CFCs all along
the route using his new detector, thereby proving that CFCs that had been
spilled/leaked/released in the northern hemisphere had evaporated and diffused
throughout the well-mixed troposphere. In 1989 the tropospheric organic
chlorine concentration was about 3.9 ppbv throughout the troposphere. Methyl
chloride (monochloromethane), the only major naturally (biologically) produce
organochlorine molecule made up about 0.6 ppbv of that total; the rest
is almost all anthropogenic.

My Radio Told Me that CFCs Can't Be an Atmospheric Problem!

CFCs not only spread throughout the troposphere, they
also diffuse into the stratosphere. The evidence for the presence of
CFCs in the stratosphere is only argued against by people who have
a vested interest

a) in suggesting that industrial chemicals are not near as damaging as
the peer-reviewed scientific evidence suggests because "industry is the
engine for the free enterprise system and what THAT produces is inherently
good"; or

b) in suggesting that scientists are dishonest leaches sucking on the government
teat and they have fabricated the evidence for the presence of CFCs in
the atmosphere so that they can increase the amount of government grant
money that would subsequently be applied to a "mythical problem"; or

c) in maintaining the ear of people who trumpet criticisms of the scientific
process, the scientific process that has proven that CFCs are a very important
component in the anthropogenic changes to the atmosphere.

This final reason is particularly insidious because it is based not on
logical reasoning but instead on efforts to try to increase the profits
(of, for instance radio stations and syndication networks) that pay nay-sayers
salaries. Scientists should speak out against popular press indictments
of well carried out scientific investigations and expose the process by
which myth, superstition, and AM radio profits are made.

Arguing against the presence of CFCs in the stratosphere is a
waste of time: CFCs have been detected in the stratosphere, their chlorine
containing decomposition products have also been detected there, and their
fluorine containing decomposition products have been detected in the stratosphere.
Period. The data
are overwhelming.

Arguing about regulating CFCs, however, though that regulation
IS presently under way, may
be worth the time.

The Smoking Gun

The presence of CFCs doesn't itself mean that they are responsible for
ozone destruction. Instead following the experimental evidence of Rowland
and Molina in the early 1970s, the smoking gun was the detection of
the photolysis products of CFC destruction in the stratosphere. That smoking
gun is chlorine monoxide (and the fluorine
containing decomposition products such as COF2 and COFCl).

Photolysis of Chlorofluorocarbons

CFCs are decomposed by high energy wavelengths of light in the upper stratosphere
after they have diffused there from the troposphere. The first chlorine
radical-freeing light reaction for CFC-12 (CF2Cl2)
can be written:

CF2Cl2 + hv (< about 260
nm) ----> Cl + CF2Cl

The stratospheric photolysis reactions ultimately lead to the complete
destruction of CFCs and the release of all the chlorine atoms the CFC contains.
Now the stage is set: Free chlorine atoms are being released into the stratosphere
from anthropogenically produced CFCs. Those CFC are ultimately decomposed
to release free chlorine atoms. And the free chlorine atoms catalytically
destroy ozone in the following way:

Catalytic Destruction of Ozone

Ozone (O3) reacts with a chlorine atom (which IS a chlorine
radical) to produce chlorine monoxide and molecular oxygen, O2.
Since atomic oxygen (O°) is also present from the natural ozone creation
cycle called the Chapman Mechanism (QuickTime
movie here--923 kb), chlorine monoxide reacts with atomic oxygen to
re-produce atomic chlorine and molecule oxygen. In this catalytic cycle
two odd-oxygen species (O3 and O) are removed and the chlorine
radical is recycled to attack another ozone molecule. The
total ozone destruction cycle looks like this:

Active Chlorine and Reservoir Species

Now if that were all there was to the stratospheric chemistry of ozone
and chlorine ALL the natural ozone layer would be destroyed. And even though
the stratospheric chlorine levels are approximately 6 times their natural
levels there are small natural levels of chlorine in the stratosphere (for
instance from biological (monochloromethane) and a small amount from volcanic
sources); therefore there must
be some natural means of removing chlorine atoms from the stratosphere.
And, there are. The presence of methane (CH4) means that chlorine
radicals can abstract a hydrogen from methane and form HCl, hydrogen chloride.
This a strong (completely dissociate) acid in aqueous solution, but in
the dry gas phase of the stratosphere hydrogen chloride exists as a stable,
undissociated molecule and is a stable "chlorine radical catcher," or reservoir
species.

Two other reactions that can form HCl are hydroxyl radical plus chlorine
monoxide or hydroperoxy radical plus chlorine radical:

OH + ClO ---> HCl + O2HO2 + Cl ----> HCl + O2

Another reservoir species is ClONO2,
somewhat misnamed as chlorine nitrate.
This stable molecule is the product of a reaction between nitrogen
dioxide and chlorine monoxide, and like hydrogen chloride, acts to sequester
active chlorine radicals from the ozone destruction cycle:

ClO + NO2 ---> ClONO2

Polar Stratospheric Clouds

OK, so here's where polar stratospheric clouds come in:

PSCs are made up of nitric acid and water crystals (most prominent for
the clouds forming at about -70 degrees C) mixed with more water containing
crystals (which form at temperatures below - 80 degrees). These clouds
have two negative effects on ozone:

they sequester oxides of nitrogen (as nitric acid) which could react with
chlorine monoxide to form the chlorine nitrate reservoir species (see immediately
above), and

they act as an active site on which a very slow gas phase reaction can
go very quickly heterogeneously (gases reacting on solid surfaces). This
reaction can be viewed as:

HCl + ClONO2 ---- on ice ----> Cl2
(gas) + HNO3 (ice)

This reaction releases molecular chlorine into the stratospheric gas phase
during the austral winter and thereby helps to remove chlorine reservoir
species. When the sun starts to shine on the polar stratosphere at the
beginning of austral spring, this chlorine gas is photolyzed back to chlorine
radicals which can THEN enter into the catalytic destruction of ozone.
The result is that at the beginning of the austral spring (~ October over
Antarctica) the ozone in the stratosphere is depleted to less than 20%
of its winter levels.

A Humble Attempt at Animating the PSC-Catalyzed Destruction
of O3

The Quicktime movie
that describes in detail the processes by which ozone is destroyed over
Antarctica by polar stratospheric clouds during the austral spring was
created especially for the students of CHM442/ESC440 during the spring
of 1997.